A federal magistrate judge has appointed a prominent specialist in federal death penalty cases to join the defense team of alleged Boston Marathon terror bomber Dzhokhar Tsarnaev.

Judy Clarke, a San Diego, Calif., attorney, has the “background, knowledge and experience” that will “enable her to provide adequate representation to the defendant,” US Magistrate Judge Marianne Bowler said in a ruling today.

Clarke’s clients have included Unabomber Ted Kaczynski; Susan Smith, who drowned her two children; and most recently Tucson, Ariz., shooter Jared Loughner. All received life sentences instead of the death penalty, The Associated Press reported.

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Bowler denied, for the time being, a request for the appointment of a second death penalty specialist for the defense team.

Tsarnaev, 19, faces charges of use of a weapon of mass destruction and malicious destruction of property resulting in death. He and his brother, Tamerlan, are accused in the April 15 bomb attacks on the marathon that killed three and injured more than 260. They also allegedly killed an MIT police officer. Tamerlan Tsarnaev, 26, died after a confrontation with police in the early morning hours of April 19. Police say they were subduing him after a shootout when his brother ran him over in a desperate escape.

Clarke joins three other attorneys — Miriam Conrad, the chief federal public defender for Massachusetts, and two assistant public defenders — in defending Tsarnaev.

The defense had asked for appointment of two attorneys “learned in the law applicable to capital cases” after Tsarnaev was charged last week.

Bowler wrote today, “A federal death penalty case implicates particular procedural requirements and ‘is extremely demanding to defend because of the effort and pressure involved.’ In light of the circumstances in this case, the defendant requires an attorney with more background, knowledge and experience in federal death penalty cases than that possessed by current counsel.”

“Appointment of Attorney Clarke is therefore justified to provide the defendant with adequate and proper representation,” Bowler wrote.

In a footnote, Bowler said, “Indeed, her qualifications demonstrate she is ‘learned in the law’ of capital cases.”

David I. Bruck, the other attorney the defense wanted to join their team, is a clinical professor of law at Washington and Lee University School of Law and director of the school’s death penalty defense clinic.

A message left at Clarke’s firm this afternoon wasn’t immediately returned. The website describes her as “specializing in complex criminal litigation, federal capital defense, and white collar crimes.” An e-mail message left for a spokeswoman for the US attorney’s office wasn’t immediately returned.